CNC Dimensional Tolerances: Limits, Function and Capability

A dimensional tolerance defines permitted size variation around a nominal dimension or between stated limits. For CNC parts, keep the required size range separate from a supplier’s claim about what its service can achieve.

Nominal size, tolerance and size limits

Nominal size provides the reference; bilateral, unilateral and limit notation communicate allowable sizes. A bilateral tolerance need not be symmetrical. These terms do not establish a machine’s capability. Protolabs tolerancing guidance

Tolerance interval and deviations

A deviation states how far a size limit lies from the stated reference dimension. The tolerance interval is the difference between the upper and lower size limits. The signs in a plus/minus callout describe the deviations; they do not mean that the full interval is the same as either one-sided allowance. Compare the resulting limits when reading bilateral, unilateral and limit notation. Callout forms

How to calculate tolerance limits

For an illustrative 20.00 ± 0.05 mm requirement:

• Lower limit: 20.00 − 0.05 = 19.95 mm.
• Upper limit: 20.00 + 0.05 = 20.05 mm.
• Total permitted size interval: 20.05 − 19.95 = 0.10 mm.

The tolerance on either side is 0.05 mm; the full interval is 0.10 mm. These are calculated drawing limits, not measured results or a supplier guarantee.

The same interval can be written as 19.95 to 20.05 mm, or as 19.95 +0.10/−0.00 mm using a different reference size. Changing 20.00 ±0.05 to 20.00 +0.10/−0.00 would shift the interval to 20.00–20.10 mm; it would not preserve the original requirement. Always compare the actual lower and upper limits.

Dimensional tolerance and geometric requirements

Size alone does not explicitly state every geometric requirement. Geometric dimensioning and tolerancing (GD&T) communicates controls for features and their relationships, including form, orientation and location. Size requirements and geometric controls can interact under the governing standard; do not assume they are independent. ASME GD&T overview

Identify the drawing standard and edition before interpreting symbols, datums or modifiers. This introductory page does not reproduce a standard or prescribe a GD&T scheme for a part.

Is there one standard tolerance for all CNC parts?

There is no single numerical default established by the sources reviewed for this page. Protolabs publishes different tolerance approaches for its automated factory and network; its German regional page also describes its own service conditions. Xometry distinguishes defaults for metal and plastic parts. These are supplier service descriptions, not a universal CNC requirement. Protolabs, German regional service, Xometry

Determine the required range from function, mating features and the actual drawing. Confirm whether the quote accepts it for the material, geometry and inspection scope. Tighter limits can change the machining process, fixturing or measurement effort. Xometry tolerance guidance

What should be checked before acceptance?

An allowable interval is a specification, not a measurement result. For the worked example, a reported 20.04 mm value lies inside the numerical limits, but that comparison alone does not settle acceptance: the measurement method and agreed decision rule still matter. Record which feature is measured and how conformity will be assessed in the quotation package.

Use the requirement in an enquiry

The next task is to document critical requirements in a CNC RFQ checklist. Surface texture describes a different aspect of the part: read Ra and Rz terminology alongside size limits. Read claim and evidence labels when evaluating a capability statement.